from collections import deque
import sys

# 入力を高速化
input = sys.stdin.read
def bfs(start, N, G, Y):
    dist = [-1] * (N + 1)
    dist[start] = Y
    que = deque([start])
    
    while que:
        pos = que.popleft()
        for to, cost in G[pos]:
            if dist[pos] >= cost and dist[pos] - cost > dist[to]:
                dist[to] = dist[pos] - cost
                que.append(to)
    
    return dist

def main():
    # 入力
    data = input().splitlines()
    N, M, P, Y = map(int, data[0].split())
    
    # グラフの初期化
    G = [[] for _ in range(N + 1)]
    edge_set = set()
    
    idx = 1
    for i in range(M):
        a, b, c = map(int, data[idx].split())
        G[a].append((b, c))
        G[b].append((a, c))
        edge_set.add((a, b))
        idx += 1
    
    # BFSの実行
    ret = bfs(1, N, G, Y)
    
    # 結果の計算
    ans = 0
    visited = set()
    for i in range(P):
        d, e = map(int, data[idx].split())
        visited.add(d)
        ans = max(ans, ret[d] // e)
        idx += 1
    
    # 結果を出力
    print(ans)

if __name__ == "__main__":
    main()